Problem 18 - Chemistry and Chemical Reactivity

Question

Place the following four compounds in order of increasing boiling point: (a) \(\mathrm{C}_{5} \mathrm{H}_{12}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{6}\) (b) \(\mathrm{CCl}_{4}\) (d) Ne

Step-by-Step Solution

Answer

Ne, C2H6, C5H12, CCl4 (increasing boiling point)

Step 1: Understanding the Compounds

Let's first identify the type of each compound. (a) \(\mathrm{C}_{5}\mathrm{H}_{12}\) is pentane, a nonpolar alkane; (b) \(\mathrm{CCl}_{4}\) is carbon tetrachloride, a nonpolar compound with a high molecular weight; (c) \(\mathrm{C}_{2}\mathrm{H}_{6}\) is ethane, a smaller alkane; (d) Neon is an inert gas. Boiling points relate to intermolecular forces among other factors.

Step 2: Analyzing Intermolecular Forces

Neon experiences weak London dispersion forces due to its monoatomic nature. \(\mathrm{C}_{2}\mathrm{H}_{6}\) experiences relatively weak London dispersion forces as well, but slightly more than neon due to its molecular nature. \(\mathrm{C}_{5}\mathrm{H}_{12}\) has more atoms and increased surface area compared to \(\mathrm{C}_{2}\mathrm{H}_{6}\), resulting in stronger London dispersion. \(\mathrm{CCl}_{4}\) is heavy and experience stronger London dispersion forces than \(\mathrm{C}_{5}\mathrm{H}_{12}\) because of its larger electron cloud size.

Step 3: Arranging Compounds by Boiling Point

As molecular weight and size often correlate with boiling point, arrange the compounds from the one with the weakest to the strongest intermolecular forces: \((d)\) neon, \((c)\) \(\mathrm{C}_{2}\mathrm{H}_{6}\), \((a)\) \(\mathrm{C}_{5}\mathrm{H}_{12}\), \((b)\) \(\mathrm{CCl}_{4}\). This arrangement reflects increasing boiling points due to increasing London dispersion forces.

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles: atoms, molecules, or ions. When we talk about boiling points, it's important to understand that these forces are a key factor in determining how much energy is needed to turn a liquid into a gas.
Boiling occurs when the intermolecular forces holding the liquid together are overcome by the energy added through heat. Fluids or substances with stronger intermolecular forces typically have higher boiling points. This is because more energy is required to break these forces and convert the liquid into vapor.

• Dipole-dipole interactions occur between polar molecules.
• Hydrogen bonding, a special case of dipole interactions, happens when hydrogen is bonded to very electronegative atoms like oxygen, nitrogen, or fluorine.
• London dispersion forces are weak and arise in all molecules, even nonpolar ones.

Different molecules have different types and strengths of intermolecular forces, which explains the diversity in boiling points.

London Dispersion Forces

London dispersion forces are weak intermolecular forces arising from temporary dipoles induced in atoms or molecules. These forces are named after Fritz London, who first proposed their existence. Despite their weakness, they are incredibly significant because they are present in all atoms and molecules.
In the original exercise, neon ( (d) ) and alkanes like ext{C}_{2} ext{H}_{6} ( (c) ) and ext{C}_{5} ext{H}_{12} ( (a) ) experience these forces. The interaction strength of London dispersion forces increases with the size of the electron cloud because larger electron clouds are more easily polarizable.

• Larger molecules tend to have stronger London dispersion forces due to more significant electron cloud distortions.
• They play a major role in nonpolar substances, influencing properties like boiling and melting points.

This explains why ext{C}_{5} ext{H}_{12} , with its larger size and electron cloud, has a higher boiling point than ext{C}_{2} ext{H}_{6} .

Molecular Weight

Molecular weight is the sum of the weights of all atoms in a molecule. It provides insight into properties like boiling and melting points. Often, higher molecular weights correlate with stronger London dispersion forces, as we see in the case of carbon tetrachloride ( ext{CCl}_{4} ) (b in the exercise), which has a relatively high molecular weight compared to the other compounds.
Larger molecules have more electrons, leading to stronger attractive forces between them. This means more energy is needed to break these forces, resulting in higher boiling points.

• Molecular weight tends to dictate how substances behave under heat, making it a critical factor in understanding boiling points.
• It helps in predicting how different substances might interact based on their size and weight.

Understanding molecular weight as a concept aids in explaining why ext{CCl}_{4} has a higher boiling point than the smaller alkanes listed in the exercise.

Alkanes

Alkanes are organic compounds consisting solely of hydrogen and carbon atoms, arranged in a tree-like structure with single bonds between carbon atoms. They are saturated hydrocarbons which means each carbon atom forms 4 bonds, either with hydrogen or other carbon atoms, maximizing their hydrogen capacity.
In the given exercise, both ext{C}_{2} ext{H}_{6} (ethane) and ext{C}_{5} ext{H}_{12} (pentane) are alkanes, showcasing typical nonpolar characteristics. These attributes affect their intermolecular forces, specifically London's dispersion forces, and therefore their boiling points.

• Alkanes have relatively lower boiling points compared to compounds with stronger intermolecular forces like hydrogen bonding.
• The longer the carbon chain (as in ext{C}_{5} ext{H}_{12} ), the higher the boiling point, due to increased surface area and stronger dispersion forces.

Understanding alkanes helps students grasp the basics of organic chemistry, especially as alkanes form the basis for more complex molecules.